Mega-sonic vibration assisted chemical mechanical planarization

ABSTRACT

A method of performing a chemical mechanical planarization (CMP) process includes holding a wafer by a retainer ring attached to a carrier, pressing the wafer against a first surface of a polishing pad, the polishing pad rotating at a first speed, dispensing a slurry on the first surface of the polishing pad, and generating vibrations at the polishing pad.

BACKGROUND

Generally, semiconductor devices comprise active components, such astransistors, formed on a substrate. Any number of interconnect layersmay be formed over the substrate connecting the active components toeach other and to outside devices. The interconnect layers are typicallymade of low-k dielectric materials comprising metallic trenches/vias.

As the layers of a device are formed, planarization processes may beperformed to planarize the layers to facilitate formation of subsequentlayers. For example, the formation of metallic features in the substrateor in a metal layer may cause uneven topography. This uneven topographymay create difficulties in the formation of subsequent layers. Forexample, uneven topography may interfere with the photolithographicprocess commonly used to form various features in a device. Therefore,it may be advantageous to planarize the surface of the device aftervarious features or layers are formed.

Chemical Mechanical Polishing (CMP) is a common practice in theformation of integrated circuits. Typically, CMP is used for theplanarization of semiconductor wafers. CMP takes advantage of thesynergetic effect of both physical and chemical forces for the polishingof wafers. It is performed by applying a load force to the back of awafer while the wafer rests on a polishing pad. A polishing pad isplaced against the wafer. Both the polishing pad and the wafer are thenrotated while a slurry containing both abrasives and reactive chemicalsis passed therebetween. CMP is an effective way to achieve globalplanarization of wafers.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the followingdetailed description when read with the accompanying figures. It isnoted that, in accordance with the standard practice in the industry,various features are not drawn to scale. In fact, the dimensions of thevarious features may be arbitrarily increased or reduced for clarity ofdiscussion.

FIG. 1 illustrates a perspective view of a chemical mechanicalplanarization apparatus, in accordance with an embodiment.

FIG. 2 illustrates a top view of the chemical mechanical planarizationapparatus of FIG. 1, in accordance with an embodiment.

FIG. 3 illustrates a cross-sectional view of the chemical mechanicalplanarization apparatus of FIG. 1, in accordance with an embodiment.

FIG. 4 illustrates a zoomed-in cross-sectional view of the polishingpad, the wafer, and the megasonic generator of the chemical mechanicalplanarization apparatus of FIG. 1, in accordance with an embodiment.

FIG. 5 illustrates a control voltage supplied to the megasonic generatorof the chemical mechanical planarization apparatus of FIG. 1, inaccordance with an embodiment.

FIG. 6 illustrates a control voltage supplied to the megasonic generatorof the chemical mechanical planarization apparatus of FIG. 1, inaccordance with another embodiment.

FIG. 7 illustrates a control voltage supplied to the megasonic generatorof the chemical mechanical planarization apparatus of FIG. 1, inaccordance with yet another embodiment.

FIG. 8 illustrates a flow chart for a method of performing a chemicalmechanical planarization process, in accordance with some embodiments.

DETAILED DESCRIPTION

The following disclosure provides many different embodiments, orexamples, for implementing different features of the invention. Specificexamples of components and arrangements are described below to simplifythe present disclosure. These are, of course, merely examples and arenot intended to be limiting. For example, the formation of a firstfeature over or on a second feature in the description that follows mayinclude embodiments in which the first and second features are formed indirect contact, and may also include embodiments in which additionalfeatures may be formed between the first and second features, such thatthe first and second features may not be in direct contact.

Further, spatially relative terms, such as “beneath,” “below,” “lower,”“above,” “upper” and the like, may be used herein for ease ofdescription to describe one element or feature's relationship to anotherelement(s) or feature(s) as illustrated in the figures. The spatiallyrelative terms are intended to encompass different orientations of thedevice in use or operation in addition to the orientation depicted inthe figures. The apparatus may be otherwise oriented (rotated 90 degreesor at other orientations) and the spatially relative descriptors usedherein may likewise be interpreted accordingly.

Embodiments of the present disclosure are described with respect tochemical mechanical planarization (CMP) tools and processes, and inparticular, CMP tools and processes that use a megasonic generatorduring the CMP process to generate vibrations at the polishing pad toreduce abrasive aggregation. In some embodiments, patterns of thevibrations, such as the frequency of the vibrations, the amplitude ofthe vibrations, and/or the duration of the vibrations, are changedduring the CMP process to achieve a target wafer polishing profile. As aresult of using the disclosed CMP tools and processes, improvedplanarity and etch rate for the CMP process are achieved. Additionaladvantage includes reduced polishing pressure used to push the wafer onthe polishing pad, which reduces wafer damage.

Chemical mechanical planarization (CMP) is a method of planarizingfeatures produced in the manufacture of semiconductor devices. Theprocess uses an abrasive material in a reactive chemical slurry inconjunction with a polishing pad. The polishing pad typically has agreater diameter than that of the semiconductor wafer. The pad and waferare pressed together during the CMP process. The process removesmaterial and tends to even out irregular topography, making the waferflat or substantially planar. This prepares the wafer for the formationof additional overlying circuit elements. For example, chemicalmechanical planarization can bring an entire wafer surface within agiven depth of field of a photolithography system. Typicaldepth-of-field specifications are on the order of, e.g., angstroms. Insome implementations, chemical mechanical planarization may also beemployed to selectively remove material based on its location on thewafer.

In a CMP process, a wafer is placed in a carrier head (also referred toas a carrier), where the wafer is held in place by a retainer ring. Thecarrier head and the wafer are then rotated as downward pressure isapplied to the wafer to press against the polishing pad. A reactivechemical solution is dispensed on a contacting surface of the polishingpad to aid planarization. The surface of a wafer may thus be planarizedusing a combination of both mechanical and chemical mechanisms.

FIG. 1 illustrates a perspective view of a chemical mechanicalplanarization apparatus 100 in accordance with some embodiments. Thechemical mechanical planarization apparatus 100 includes a platen 105and a polishing pad 115 over (e.g., glued to) the platen 105. In someembodiments, the polishing pad 115 includes a single layer or acomposite layer of materials, such as felts, polymer impregnated felts,microporous polymer films, microporous synthetic leathers, filledpolymer films, unfilled textured polymer films, combinations thereof, orthe like. Representative polymers include polyurethane, polyolefins, orthe like.

As illustrated in FIG. 1, a polisher head 120 (may also be referred toas a polishing head) is placed over the polishing pad 115. The polisherhead 120 includes a carrier 125, a retainer ring 127, and a megasonicgenerator 320. The retainer ring 127 and the megasonic generator 320 aremounted to the carrier 125 using mechanical fasteners (e.g., screws, orthe like) or other suitable attachment means. In the example of FIG. 1,the retainer ring 127 is attached to a lower side of the carrier 125 andthe megasonic generator 320 is attached to an upper side of the carrier125. The megasonic generator 320 may comprise a piezoelectric transducer(PZT) and is configured to generate vibrations at the carrier 125 and atthe polishing pad 115 during the CMP process. More details regarding themegasonic generator 320 are discussed hereinafter.

During a representative chemical mechanical planarization process, aworkpiece (e.g., a semiconductor wafer; not shown in FIG. 1 butillustrated and described below with respect to FIG. 3) is placed withinthe carrier 125 and is held by the retainer ring 127. In someembodiments, the retainer ring 127 has a substantially annular shapewith a substantially hollow center. The workpiece is placed in thecenter of the retainer ring 127 such that the retainer ring 127 holdsthe workpiece in place during a chemical mechanical planarizationprocess. The workpiece is positioned such that a surface to be polishedfaces in a direction (for example, downward) towards the polishing pad115. The carrier 125 is configured to apply downward force or pressureurging the workpiece into contact with the polishing pad 115. Thepolisher head 120 is configured to rotate the workpiece over thepolishing pad 115 during the chemical mechanical planarization process,thereby imparting mechanical abrading action to affect planarization orpolishing of a contacting surface of the workpiece.

In some embodiments, the chemical mechanical planarization apparatus 100includes a slurry dispenser 140 configured to deposit a slurry 150 ontothe polishing pad 115. The platen 105 is configured to rotate, causingthe slurry 150 to be distributed between the workpiece and the platen105 through a plurality of grooves in the retainer ring 127. Theplurality of grooves may extend from an outer sidewall of the retainerring 127 to an inner sidewall of the retainer ring 127.

The composition of the slurry 150 may depend on which types of materialare to be polished or removed. For example, the slurry 150 may comprisea reactant, an abrasive, a surfactant, and a solvent. The reactant maybe a chemical, such as an oxidizer or a hydrolyzer, which chemicallyreacts with a material of the workpiece in order to assist the polishingpad 115 in abrading or removing material. In some embodiments in whichthe material to be removed includes, e.g., tungsten, the reactant maybe, e.g., hydrogen peroxide, Cr2O7, MnO4, OsO4; although other suitablereactants, such as hydroxylamine, periodic acid, other periodates,iodates, ammonium persulfate, peroxomonosulfates, peroxymonosulfuricacid, perborates, malonamide, combinations of same, or the like, thatare configured to aid removal of material may be alternatively,conjunctively, or sequentially employed. In other embodiments, otherreactants may be used to remove other types of materials. For example,in embodiments in which a material to be removed includes, e.g., anoxide, the reactant may comprise, e.g., nitric acid (HNO₃), potassiumhydroxide (KOH), ammonium hydroxide (NH₄OH), combinations thereof, orthe like.

The abrasive may include any suitable particulate that, in conjunctionwith the relative mechanical movement of the polishing pad 115, isconfigured to polish or planarize the workpiece. In some embodiments,the abrasive includes colloidal aluminum oxide. In some embodiments, theabrasive includes silicon oxide, aluminum oxide, cerium oxide,polycrystalline diamond, polymer particles (e.g., polymethacrylate, orthe like), combinations thereof, or the like.

The surfactant may be utilized to help disperse the reactant(s) andabrasive(s) within the slurry 150, and to prevent (or otherwise reducethe occurrence of) agglomeration of the abrasive during the chemicalmechanical planarization process. In some embodiments, the surfactantmay include polyethylene glycol (PEG), polyacrylic acid, sodium salts ofpolyacrylic acid, potassium oleate, sulfosuccinates, sulfosuccinatederivatives, sulfonated amines, sulfonated amides, sulfates of alcohols,alkylanyl sulfonates, carboxylated alcohols, alkylamino propionic acids,alkyliminodipropionic acids, potassium oleate, sulfosuccinates,sulfosuccinate derivatives, sulfates of alcohols, alkylanyl sulfonates,carboxylated alcohols, sulfonated amines, sulfonated amides, alkylaminopropionic acids, alkyliminodipropionic acids, combinations thereof, orthe like. However, such representative embodiments are not intended tobe limited to the recited surfactants. Those skilled in the art willappreciate that any suitable surfactant may be alternatively,conjunctively, or sequentially employed.

In some embodiments, the slurry 150 includes a solvent that may beutilized to combine the reactant(s), the abrasive(s), and thesurfactant(s), and allow the mixture to be moved and dispersed onto thepolishing pad 115. In some embodiments, the solvent includes, e.g.,deionized water (DIW), alcohol, or an azeotropic mixture thereof;however, other suitable solvent(s) may be alternatively, conjunctively,or sequentially employed.

Additionally, if desired, other additives may also be added in order tohelp control or otherwise benefit the CMP process. For example, acorrosion inhibitor may be added in order to help control the corrosion.In one particular embodiment the corrosion inhibitor may be an aminoacid such as glycine. However, any suitable corrosion inhibitor may beutilized.

In another embodiment, a chelating agent(s) is added to the slurry 150.The chelating agent may be an agent such as ethylenediaminetetraaceticacid (EDTA), C₆H₈O₇, C₂H₂O₄, combinations thereof, or the like. However,any suitable chelating agent may be utilized.

In yet another embodiment, the slurry 150 includes a pH adjuster(s) inorder to control the pH value of the slurry 150. For example, a pHadjuster such as HCl, HNO₃, H₃PO₄, C₂H₂(COOH)₂, KOH, NH₄OH, combinationsthereof, or the like, may be added to the slurry 150 in order to adjustthe pH value of the slurry 150 up or down.

Additionally, other additives may also be added to help control andmanage the CMP process. For example, down-force enhancers (e.g., anorganic compound), polish rate inhibitors, or the like may also beadded. Any suitable additives which may be useful to the polishingprocess may be utilized, and all such additives are fully intended to beincluded within the scope of the embodiments.

In some embodiments, the chemical mechanical planarization apparatus 100includes a pad conditioner 137 attached to a pad conditioner head 135.The pad conditioner head 135 is configured to rotate the pad conditioner137 over the polishing pad 115. The pad conditioner 137 is mounted tothe pad conditioner head 135 using mechanical fasteners (e.g., screws,or the like) or by other suitable attachment means. A pad conditionerarm 130 is attached to the pad conditioner head 135, and is configuredto move the pad conditioner head 135 and the pad conditioner 137 in asweeping motion across a region of the polishing pad 115. In someembodiments, the pad conditioner head 135 is mounted to the padconditioner arm 130 using mechanical fasteners (e.g., screws, or thelike) or by other suitable attachment means. The pad conditioner 137comprises a substrate over which an array of abrasive particles isbonded. The pad conditioner 137 removes built-up wafer debris and excessslurry 150 from the polishing pad 115 during the CMP processing. In someembodiments, the pad conditioner 137 also acts as an abrasive for thepolishing pad 115 to renew, or create a desired texture (such as, e.g.,grooves, or the like) against which the workpiece may be polished.

As illustrated in FIG. 1, the chemical mechanical planarizationapparatus 100 has a single polisher head (e.g., 120) and a singlepolishing pad (e.g., 115). However, in other embodiments, the chemicalmechanical planarization apparatus 100 may have multiple polisher headsor multiple polishing pads. In some embodiments in which the chemicalmechanical planarization apparatus 100 has multiple polisher heads and asingle polishing pad, multiple workpieces (e.g., semiconductor wafers)may be polished at a same time. In other embodiments in which thechemical mechanical planarization apparatus 100 has a single polisherhead and multiple polishing pads, a chemical mechanical planarizationprocess may include a multi-step process. In such embodiments, a firstpolishing pad may be used for bulk material removal from a wafer, asecond polishing pad may be used for global planarization of the wafer,and a third polishing pad may be used, e.g., to buff a surface of thewafer. In some embodiments, different slurry compositions may be usedfor different stages of chemical mechanical planarization processing. Instill other embodiments, a same slurry composition may be used for allchemical mechanical planarization stages.

FIG. 2 illustrates a top view (or plan view) of the chemical mechanicalplanarization apparatus 100 of FIG. 1, in accordance with someembodiments. The platen 105 (located beneath the polishing pad 115 inFIG. 2) is configured to rotate in a clockwise or a counter-clockwisedirection, indicated by a double-headed arrow 215 around an axisextending through a centrally-disposed point 200, which is a centerpoint of the platen 105. The polisher head 120 is configured to rotatein a clockwise or a counter-clockwise direction, indicated by adouble-headed arrow 225 around an axis extending through a point 220,which is a center point of the polisher head 120. The axis through thepoint 200 is parallel to the axis through the point 220. In theillustrated embodiment, the axis through the point 200 is spaced apartfrom the axis through the point 220. The pad conditioner head 135 isconfigured to rotate in a clockwise or a counter-clockwise direction,indicated by a double-headed arrow 235 around an axis extending througha point 230, which is a center point of the pad conditioner head 135.The axis through the point 200 is parallel to the axis through the point230. The pad conditioner arm 130 is configured to move the padconditioner head 135 in an effective arc during rotation of the platen105, as indicated by a double-headed arrow 237.

As features size continues to shrink in advanced semiconductorprocessing nodes, the requirement for planarity of the various layers onthe wafer becomes more stringent. In some advanced technology nodes,nanometer sized abrasives are used in the slurry of the CMP process. Thesize (e.g., diameter) of the nanometer sized abrasives (also referred toas nano-particles, or nano-abrasive particles) may be smaller than about30 nm, such as between about 3 nm and about 5 nm. A slurry using thenano-abrasive particles is also referred to as a nano-abrasive slurry.In contrast, the size (e.g., diameter) of the abrasives in aconventional slurry may be larger than 35 nm, such as between about 50nm about 100 nm.

While a CMP process using the nano-abrasive slurry may achieve betterplanarity, many challenges exist. For example, if a CMP process isperformed by simply replacing the conventional slurry with anano-abrasive slurry, the etch rate (also referred to as removal rate)of the CMP process using the nano-abrasive slurry may be very slow, suchas less than about 200 angstroms per minute. Such a slow etch rate maybe impractical for use in manufacturing, due to the long CMP processingtime required. To compensate for the slow etch rate using thenano-abrasive slurry, conventional CMP process may have to increase theforce/pressure (may be referred to as polishing pressure hereinafter foreasy of discussion) that is used to press the wafer against thepolishing pad 115, or increase the flow rate of slurry used in the CMPprocess. However, increasing the polishing pressure may increase therisk of wafer damage, such as scratches or cracks in the wafer.Increasing the polishing pressure may also make it difficult for theslurry to flow between the polishing pad 115 and the wafer, which maycause unwanted behaviors for the polishing process. Furthermore,increasing the flow rate of slurry increases the consumption of slurry,which increases the manufacturing cost.

Another challenge for CMP processes using the nano-abrasive slurry isabrasive aggregation, which refers to the issue that abrasives in theslurry are not distributed evenly across the surface of the polishingpad 115 and may aggregate at certain locations, such as in openings 116(see FIG. 4) at the upper surface of the polishing pad 115.

Referring temporarily to FIG. 4, which shows a cross-sectional view of(portions of) polishing pad 115, wafer 300, and megasonic generator 320.As illustrated in FIG. 4, the polishing pad 115 may be porous and mayhave cavities 115H therein. The openings 116 may be formed, at least inpart, by cavities exposed at the upper surface of the polishing pad 115.FIG. 4 also illustrates the abrasives 410 used in the slurry, whichabrasives 410 may aggregate in the openings 116, instead of on the topsurfaces of the peaks 115P (e.g., protruding portions at the uppersurface) of the polishing pad 115. During the CMP process, the abrasives410 in the openings 116 may not come into contact with the wafer 300,therefore are ineffective, which lowers the effectiveness of the slurryand results in lowered etch rate of the CMP process. On the other hand,if abrasives 410 are aggregated at certain locations at the top surfacesof the peaks 115P, over-etching of those locations may occur, which mayresult in local dishing on the surface of the wafer 300. Variousembodiments discussed herein prevents or reduces abrasive aggregation byusing the megasonic generator 320, thereby achieving improved etch rateand better surface planarity for CMP processes. In addition, thedisclosed embodiments allow for lower polishing pressure and lower flowrate for slurry to be used in the CMP process, thereby reducing the riskof wafer damage and lowering the manufacturing cost, details of whichare discussed hereinafter.

Referring now to FIG. 3, which illustrates a cross-sectional view of thechemical mechanical planarization apparatus 100 of FIG. 1, in accordancewith an embodiment. Note that for clarity, not all features of thechemical mechanical planarization apparatus 100 are illustrated in FIG.3. As illustrated in FIG. 3, the platen 105, with the polishing pad 115attached on top, rotates around an axis 106. The polisher head 120includes the carrier 125, the retainer ring 127, and the megasonicgenerator 320, and rotates around an axis 327. The platen 105 and thepolisher head 120 may rotation in a same direction, or in oppositedirections.

In the example of FIG. 3, the carrier 125 includes a membrane 310configured to interface with a wafer 300 during the CMP process. In someembodiments, the chemical mechanical planarization apparatus 100includes a vacuum system coupled to the polisher head 120, and themembrane 310 is configured to pick up and hold the wafer 300 onto themembrane 310 using, e.g., vacuum suction. The membrane 310 may form anenclosed space by itself or with a lower side of the carrier 125. Duringthe CMP process, a pressure (may also be referred to as an interiorpressure of the membrane) within the enclosed space may be maintained ata pre-determined level, such that the inflated membrane 310 pushes thewafer 300 down toward the polishing pad 115. By adjusting the interiorpressure of the membrane 310, the polishing pressure may be adjusted.

Still referring to FIG. 3, the megasonic generator 320 includes a holder321, electrical terminals 325, and a piezoelectric transducer (PZT) 323.The holder 321 may be used to hold the PZT 323 and to attach themegasonic generator 320 to the axis 327. The holder 321 may also includecircuits that electrically couple the electrical terminals 325 with thePZT 323. A control voltage Vt, which is used to control the operationsof the PZT 323 to generate vibrations, is applied to the electricalterminals 325 by a power supply 331. The power supply 331 may include acontrollable voltage source and a power amplifier to generate thecontrol voltage Vt at output terminals 333. The control voltage at theoutput terminals 333 of the power supply 331 is then supplied to theelectrical terminals 325 by conductive lines 340 (e.g., copper lines).In some embodiments, the conductive lines 340 are routed through theinterior of the axis 327 (which may be hollow) to connect with the powersupply 331.

FIG. 3 further illustrates a controller 335 electrically coupled to thepower supply 331. The controller 335 may instruct and control the powersupply 331 to generate control voltages with different parameters togenerate different vibration patterns by the PZT 323, details of whichare discussed below with reference to FIGS. 5-7. In some embodiments,the power supply 331 and the controller 335 are external to, thus not apart of, the megasonic generator 320. In some embodiments, the powersupply 331 and the controller 335 are integrated into the megasonicgenerator 320, thus are part of the megasonic generator 320.

As illustrated in FIG. 3, the PZT 323 is attached to the carrier 125.When the control voltage Vt is applied to the PZT 323 during the CMPprocess, movement of the PZT 323 generates vibrations which, throughphysical contact or other transmission media (e.g., slurry), aretransmitted to, e.g., the carrier 125, the retainer ring 127, and thepolishing pad 115. The vibrations generated by the megasonic generator320 may be along a first direction within a plane parallel to the uppersurface of the polishing pad 115 (e.g., a plane defined by the x-axisand the y-axis in FIG. 1), along a second direction (e.g., along thez-axis in FIG. 1) perpendicular to the upper surface of the polishingpad 115, or along both the first direction and the second direction.Although the megasonic generator 320 is illustrated in FIG. 3 as beingmounted on the carrier 125 and rotating around the axis 327, otherconfigurations or locations for the megasonic generator 320 are possibleand are fully intended to be included within the scope of the presentdisclosure. For example, the megasonic generator 320 may be mounted atthe lower surface of the platen 105, and may rotate around the axis 106.

In some embodiments, the wafer 300 is a semiconductor wafer comprising,for example, a semiconductor substrate (e.g., comprising silicon, aIII-V semiconductor material, or the like), active devices (e.g.,transistors, or the like) formed in or on the semiconductor substrate,and various interconnect structures. Representative interconnectstructures may include conductive features, which electrically connectthe active devices to form functional circuits. In various embodiments,the CMP process may be applied to the wafer 300 during any stage ofmanufacture in order to planarize features or otherwise remove material(e.g., dielectric material, semiconductor material, conductive material,or the like) of the wafer 300. The wafer 300 may include any subset ofthe above-identified features, as well as other features.

As illustrated in FIG. 3, the wafer 300 comprises bottommost layer(s)305 and overlying layer(s) 307. The bottommost layer 305 is subjected topolishing/planarization during a CMP process. In some embodiments, thebottommost layer 305 comprises metal, such as tungsten, copper, cobalt,titanium, ruthenium, combinations thereof, or the like. In someembodiments, the bottommost layer 305 comprises a dielectric material,such as silicon oxide, silicon nitride, combinations thereof, or thelike. In some embodiments, the bottommost layer 305 comprises asemiconductor material, such as silicon, polysilicon, silicon germanium,silicon carbide, the combinations thereof, or the like. The bottommostlayer 305 may be polished to form, e.g., contact plugs contactingvarious active devices of the wafer 300. In embodiments in which thebottommost layer 305 comprises copper, the bottommost layer 305 may bepolished to form, e.g., various interconnect structures of the wafer300. In embodiments in which the bottommost layer 305 comprises adielectric material, the bottommost layer 305 may be polished to form,e.g., shallow trench isolation (STI) structures on the wafer 300.

In some embodiments, the bottommost layer 305 may have a non-uniformthickness (e.g., exhibiting local or global topological variation of anexposed surface of the bottommost layer 305) resulting from processvariations experienced during deposition of the bottommost layer 305.For example, in an embodiment in which the bottommost layer 305 beingplanarized comprises tungsten, the bottommost layer 305 may be formed bydepositing tungsten into an opening through a dielectric layer using achemical vapor deposition (CVD) process. Due to CVD process variationsor other underlying structures, the bottommost layer 305 may have anon-uniform thickness.

In some embodiments, a thickness profile of the bottommost layer 305 maybe measured using ellipsometry, interferometry, reflectometry,picosecond ultrasonics, atomic force microscopy (AFM), scanningtunneling microscopy (STM), scanning electron microscopy (SEM),transmission electron microscopy (TEM), or the like. In someembodiments, a thickness measurement apparatus (not shown) may beexternal to the chemical mechanical planarization apparatus 100, and athickness profile of the bottommost layer 305 may be measured orotherwise determined before loading the wafer 300 into the chemicalmechanical planarization apparatus 100. In other embodiments, athickness measurement apparatus may be a part of the chemical mechanicalplanarization apparatus 100, and a thickness profile of the bottommostlayer 305 may be measured or otherwise determined after loading thewafer 300 into the chemical mechanical planarization apparatus 100.

After measurement, the bottommost layer 305 may be planarized by thechemical mechanical planarization apparatus 100. In a particularembodiment the polisher head 120 may be lowered such that the bottommostlayer 305 of the wafer 300 is in physical contact with the polishing pad115. Additionally, the slurry 150 is also introduced onto the polishingpad 115, such that the slurry 150 will come into contact with theexposed surfaces of the bottommost layer 305. For example, the slurry150 may be introduced at a flow rate of between about 100 cubiccentimeters per minute (cc/min) and about 500 cc/min, such as about 250cc/min. The surface (e.g., the bottommost layer 305) of the wafer 300may thus be planarized using a combination of both mechanical andchemical forces.

Referring now to FIG. 4, which illustrates a zoomed-in cross-sectionalview of a portion of the polishing pad 115, a portion of the wafer 300,and a portion of the megasonic generator 320. During the CMP process,the vibrations generated by the megasonic generator 320 helps todistribute the abrasives 410 evenly across the upper surface of thepolishing pad 115. For example, some of the abrasives 410 aggregated atthe bottom of the openings 116 (e.g., between adjacent peaks 115P) maybe stirred up by the vibrations and get distributed to the top surfacesof the peaks 115P (e.g., between the peaks 115P and the wafer 300), thusbecoming effective abrasives 410 that participate in the polishingprocess. As a result, the etch rate of the CMP process may reach, e.g.,3000 A angstroms per minute or larger for an oxide film, which may be10% to 20% better than a CMP process without using the megasonicgenerator 320. The even distribution of the abrasives 410 also reducesthe local dishing effect, thus achieving a better planarity for thepolished wafer surface.

In some embodiments, a frequency of the vibration generated by themegasonic generator 320, which may be the same as, or proportional to,the frequency of the control voltage Vt, is between about 10 KHz andabout 50 KHz. A vibration frequency smaller than 10 KHz may be too lowand not effective for reducing abrasive aggregation. A vibrationfrequency larger than 50 KHz, however, may be too high and may damagethe wafer 300 (e.g., may damage the surface of the wafer 300).

In some embodiments, a rotational speed of the platen 105 is betweenabout 30 rounds per minute (rpm) and about 120 rpm, and a rotationalspeed of the polisher head 120 is between about 30 rpm and about 120rpm. If the rotational speeds of the platen 105 and the polisher head120 are below about 30 rpm, the rotational speeds may be too low, andthe effect of the vibrations may be limited to local regions of thewafer surface for too long, and may cause uneven etch rates in differentlocal regions of the wafer surface. For example, the abrasive 410 may bestirred up from the bottom of the openings 116 but may not bedistributed to other regions quickly enough, which may create localizedsurface regions with higher concentrations of abrasives than otherlocalized surface regions with less openings 116. Therefore, the etchrate at different localized regions may be different and may cause theuneven etch rates. On the other hand, if the rotational speeds of theplaten 105 and the polisher head 120 are above about 120 rpm, theeffectiveness of the slurry may decrease. This may be due to freshslurry being spread out too quickly over the pad surface, which mayresult in a very thin layer of fresh slurry, thereby reducing the etchrate of the CMP process.

In some embodiments, the porosity of the polishing pad 115 is betweenabout 10% and about 80%, such as between about 30% and about 60%, orbetween about 40% and about 50%. For polishing pad 115 with lowerporosity (e.g., <10%), the benefit of the megasonic generator 320 maynot be significant enough to justify the cost of the megasonic generator320, since there is very little abrasive aggregation in the openings116. If the porosity is too high (e.g., >80%), the effectiveness of themegasonic generator 320 may be limited. This is because with highconcentrations of openings 116 at the upper surface of the polishing pad115, the total area of the top surfaces of the peaks 115P, where theeffective abrasives 410 rest during the CMP process, is too limited. Inother words, an ineffective abrasive particle 410, stirred up from thebottom of an opening 116 by the vibrations, may likely fall back intoanother opening 116, thus remains an ineffective abrasive particle.

Due to the improved etch rate achieved by the disclosed embodiment, theflow rate of the slurry and the polishing pressure do not need to beincreased. In some embodiments, the flow rate of the slurry is betweenabout 100 cc/min and about 500 cc/min, such as about 250 cc/min. The lowflow rate of slurry allowed for by the present disclosure reducesmanufacturing cost associated with the consumption of slurry. In someembodiments, due to the low polishing pressure allowed for by thepresent disclosure, the interior pressure of the membrane 310 may be setbetween about 0.5 psi and about 3 psi during the CMP process, which islower than a range between about 1 psi and about 5 psi used for aconventional CMP process without the megasonic generator 320. The lowerpolishing pressure reduces wafer damage. In addition, the use ofmegasonic generator 320 in the CMP process also achieves better surfaceplanarity than the conventional CMP process. For example, the topography(e.g., unevenness) of the polished wafer surface using the disclosedembodiments is about 10% to about 50% of the topography of the polishedwafer surface using the conventional CMP process.

FIG. 5 illustrates a control voltage supplied to the megasonic generator320, in an embodiment. The x-axis of FIG. 5 illustrates time, and they-axis illustrates amplitude. The control voltage of FIG. 5 is acontinuous wave (e.g., a sine wave, or a cosine wave) signal with apre-determined frequency between, e.g., about 10 KHz and about 50 KHz.The frequency of the control voltage is the same as, or proportional to,the vibration frequency of the megasonic generator 320, in theillustrated embodiment. In some embodiments, the frequency of thecontrol voltage is adjusted to achieve a target polished surface profileafter the CMP process. Recall that the thickness profile of thebottommost layer 305 of the wafer 300 may be measured before the CMPprocess. The measured thickness profile of the bottommost layer 305 maybe used to determine, e.g., the frequency of the control voltage for themegasonic generator 320. In some embodiments, the amplitude of thecontrol voltage is adjusted to achieve a target polished surface profileafter the CMP process. In some embodiments, both the frequency and theamplitude of the control voltage are adjusted to achieve a targetpolished surface profile after the CMP process.

FIG. 6 illustrates a control voltage supplied to the megasonic generator320, in another embodiment. The x-axis of FIG. 6 illustrates time, andthe y-axis illustrates amplitude. The control voltage in FIG. 6comprises bursts of control voltage, e.g., 610A, 610B, where each burstof control voltage includes one or more cycles (e.g., periods) of acontinuous-wave control voltage same as or similar to the controlvoltage of FIG. 5. For example, each burst of control voltage may have aduration between about 1 ms to about 300 ms. In some embodiments, eachburst of control voltage has a first frequency, and generates vibrationswith a second frequency at the megasonic generator 320, where the secondfrequency is the same as or proportional to the first frequency.

In the example of FIG. 6, the amplitude, the frequency, and/or theduration of each burst of control voltage (e.g., 610A, or 610B) may beadjusted individually to achieve a target polished surface profile, thusdifferent from the amplitude, the frequency, and/or the duration ofanother burst of control voltage. As illustrated in FIG. 6, each burstof control voltage is separated from an adjacent burst of controlvoltage by a period of silence (e.g., a period of time with no controlvoltage or zero control voltage, which corresponds to no vibration beinggenerated at the megasonic generator 320). In some embodiments, thedurations (see, e.g., T_(A) and T_(B) in FIG. 6) between adjacent burstsof control voltages may be adjusted individually, and therefore, aredifferent from each other. In the example of FIG. 6, the controlvoltages may include bursts of control voltage with large amplitudes(e.g., 610A, 610C, 610D), and bursts of control voltages with smalleramplitudes (e.g., 610B, 610E) that are interspersed between the burstsof control voltage with large amplitudes. Each of the bursts of controlvoltage with large amplitudes (e.g., 610A, 610C, or 610D) may have anoffset (e.g., a non-zero mean value) in its amplitude, and therefore,may include one or more cycles of a continuous-wave control voltageoscillating around the non-zero mean value. For example, the controlvoltage 610A has a positive offset, and the control voltage 610C has anegative offset. In addition, the bursts of control voltage with largeamplitudes (e.g., 610A, 610C, or 610D) may have alternating signs (e.g.,positive or negative) for their offsets, in some embodiments. Each ofthe bursts of control voltages with smaller amplitudes (e.g., 610B,610E) in FIG. 6 may have a zero mean value, or may have a small positiveoffset or a small negative offset that is less than, e.g., 10% of theoffset of the large control voltages (e.g., 610A, 610C, 610D). Thepatterns of the control voltages illustrated in FIG. 6 are merelynon-limiting examples, other patterns are also possible and are fullyintended to be included within the scope of the present disclosure.

The control voltage illustrated in FIG. 6 allows for many degrees offreedom in fine tuning the CMP process to achieve a target polishedsurface profile. One skilled in the art will appreciate that while thediscussion above describes each burst of control voltage as beingadjusted individually, the embodiment where some, or all, of the burstsof control voltage share the same parameters (e.g., amplitude,frequency, duration of each burst of control voltage, and/or durationbetween adjacent bursts of control voltage) are fully intended to beincluded within the scope of the present disclosure.

FIG. 7 illustrates another embodiment control voltage, where the controlvoltage is changed dynamically at different stages of a CMP process. Thex-axis of FIG. 7 illustrates time, and the y-axis illustrates frequency.In the illustrated embodiment, the frequency of the control voltage isthe same as or proportional to the vibration frequency at the megasonicgenerator 320. In FIG. 7, between time T0 and T1, the CMP process is inthe ramp-up stage, and the frequency of the control voltage is ramped upin preparation for entering the main polish stage of the CMP process.Between time T1 and time T2, the CMP process is in the main polishstage, where a first control voltage frequency (e.g., between about 20KHz and about 26 KHz) is used to polish the wafer at a relatively highetch rate. Between time T2 and time T3, the CMP process enters thebuffing polish step, where a second control voltage frequency (e.g.,between about 5 KHz and about 15 KHz) smaller than the first controlvoltage frequency is used to buff the wafer at a slower etch rate. Aftertime T3, the CMP process enters the de-chuck stage, where the wafer isgetting ready to be removed from the retainer ring 127. A third controlvoltage frequency (e.g., smaller than 4 KHz), which may be smaller thanthe second control voltage frequency, is used during the de-chuck stage.The example of FIG. 7 further illustrates a temporary decrease in thethird control voltage frequency after time T3 for, e.g., about 3 to 5seconds, before the third control voltage frequency settles at about 4KHz. The temporary decrease (e.g., with a control voltage frequency atabout 2 KHz) may be implemented to accommodate the polishing head, whichmay be unstable at the beginning of the de-chuck stage.

Embodiment may achieve advantages. For example, the megasonic generatorreduces abrasive aggregation and helps to evenly distribute theabrasives along the surface of the polishing pads, thereby improving theetch rate of the CMP process and achieving a better planarity for thepolished wafer surface. The disclosed CMP tool allows a low polishingpressure (e.g., between about 0.5 psi and about 3 psi) to be applied tothe wafer, thereby reducing risks of wafer damage related to largepolishing pressure. The flow rate of the slurry used in the CMP processmay be kept low compared with a conventional CMP process, which savesmanufacturing cost associated with slurry consumption. Although thepresent disclosure is discussed using the example of nano-abrasiveslurry, the disclosed embodiments may be applied to CMP processes usingabrasives having other sizes, such as having diameters between 2 nm andabout 300 nm.

FIG. 8 illustrates a flow chart for a method of performing a chemicalmechanical planarization process, in accordance with some embodiments.It should be understood that the embodiment method shown in FIG. 8 ismerely an example of many possible embodiment methods. One of ordinaryskill in the art would recognize many variations, alternatives, andmodifications. For example, various steps as illustrated in FIG. 8 maybe added, removed, replaced, rearranged and repeated.

Referring to FIG. 8, at step 1010, a wafer is held by a retainer ringattached to a carrier. At step 1020, the wafer is pressed against afirst surface of a polishing pad, the polishing pad rotating at a firstspeed. At step 1030, a slurry is dispensed on the first surface of thepolishing pad. At step 1040, vibrations are generated at the polishingpad.

In accordance with an embodiment, a chemical mechanical planarization(CMP) tool includes a carrier; a retainer ring attached to the carrierand configured to hold a wafer during a CMP process; and a megasonicgenerator attached to the carrier and configured to generate vibrationsduring the CMP process. In an embodiment, the megasonic generatorcomprises a piezoelectric transducer. In an embodiment, the megasonicgenerator is configured to generate the vibrations having a frequencybetween about 10 KHz and about 50 KHz. In an embodiment, the megasonicgenerator is configured to generate the vibrations along directionswithin a plane parallel to a major surface of the wafer, or along adirection perpendicular to the major surface of the wafer. In anembodiment, the CMP further includes a platen; and a polishing padattached to an upper surface of the platen, where the carrier isconfigured to press the wafer against the polishing pad during the CMPprocess. In an embodiment, a porosity of the polishing pad is betweenabout 10% and about 80%. In an embodiment, the CMP tool further includesa slurry dispenser, where the slurry dispenser is configured to dispensea slurry on the polishing pad during the CMP process, where a diameterof abrasives in the slurry is smaller than about 30 nm.

In accordance with an embodiment, a method of performing a chemicalmechanical planarization (CMP) process includes rotating a polishing padat a first rotational speed; dispensing a slurry on a first surface ofthe polishing pad; pressing a wafer against the first surface of thepolishing pad, the wafer being held by a retaining ring of a carrier;and generating vibrations at the polishing pad during the CMP processusing a megasonic generator. In an embodiment, the first rotationalspeed is between about 30 round per minute (rpm) and about 120 rpm,where the method further comprises rotating the wafer at a secondrotational speed between about 30 rpm and about 120 rpm. In anembodiment, generating vibrations comprises generating vibrations havinga frequency between about 10 KHz and about 50 KHz using the megasonicgenerator. In an embodiment, a porosity of the polishing pad is betweenabout 10% and about 80%. In an embodiment, a diameter of abrasives inthe slurry is smaller than about 30 nm. In an embodiment, dispensing theslurry comprises dispensing the slurry on the first surface of thepolishing pad during the CMP process at a flow rate between about 0.1liter per minute and about 0.5 liter per minute. In an embodiment, thecarrier comprises a membrane that is in contact with the wafer duringthe CMP process, where pressing the wafer comprises inflating themembrane at a pre-determined pressure level to press the wafer againstthe first surface of the polishing pad, the pre-determined pressurelevel being between about 0.5 pound per square inch (psi) and about 3psi. In an embodiment, generating vibrations comprises: generating thevibrations at a first vibration frequency during a main polish step ofthe CMP process; generating the vibrations at a second vibrationfrequency smaller than the first vibration frequency during a buffingpolish step of the CMP process; and generating the vibrations at a thirdvibration frequency smaller than the second vibration frequency during ade-chuck step of the CMP process. In an embodiment, generating thevibrations comprises generating a first burst of vibrations and a secondburst of vibrations separated from the first burst of vibrations by aperiod of time with no vibration, where a first amplitude of the firstburst of vibrations is different from a second amplitude of the secondburst of vibrations.

In accordance with an embodiment, a method of performing a chemicalmechanical planarization (CMP) process includes holding a wafer by aretainer ring attached to a carrier; pressing the wafer against a firstsurface of a polishing pad, the polishing pad rotating at a first speed;dispensing a slurry on the first surface of the polishing pad; andgenerating vibrations at the polishing pad. In an embodiment, generatingthe vibrations comprises generating the vibrations with a vibrationfrequency between about 10 KHz and about 50 KHz using a megasonicgenerator attached to the carrier, the megasonic generator comprising apiezoelectric transducer. In an embodiment, the first speed is betweenabout 30 round per minute (rpm) and about 120 rpm, and the methodfurther comprises rotating the carrier at a second speed between about30 rmp and about 120 rpm. In an embodiment, generating the vibrationscomprises generating the vibrations with a first vibration frequencyduring a first stage of the CMP process, and generating the vibrationswith a second vibration frequency different from the first vibrationfrequency during a second stage of the CMP process.

The foregoing outlines features of several embodiments so that thoseskilled in the art may better understand the aspects of the presentdisclosure. Those skilled in the art should appreciate that they mayreadily use the present disclosure as a basis for designing or modifyingother processes and structures for carrying out the same purposes and/orachieving the same advantages of the embodiments introduced herein.Those skilled in the art should also realize that such equivalentconstructions do not depart from the spirit and scope of the presentdisclosure, and that they may make various changes, substitutions, andalterations herein without departing from the spirit and scope of thepresent disclosure.

What is claimed is:
 1. A chemical mechanical planarization (CMP) toolcomprising: a carrier; a retainer ring attached to the carrier andconfigured to hold a wafer during a CMP process; and a megasonicgenerator attached to the carrier and configured to generate vibrationsduring the CMP process.
 2. The CMP tool of claim 1, wherein themegasonic generator comprises a piezoelectric transducer.
 3. The CMPtool of claim 1, wherein the megasonic generator is configured togenerate the vibrations having a frequency between about 10 KHz andabout 50 KHz.
 4. The CMP tool of claim 3, wherein the megasonicgenerator is configured to generate the vibrations along directionswithin a plane parallel to a major surface of the wafer, or along adirection perpendicular to the major surface of the wafer.
 5. The CMPtool of claim 3, further comprising: a platen; and a polishing padattached to an upper surface of the platen, wherein the carrier isconfigured to press the wafer against the polishing pad during the CMPprocess.
 6. The CMP tool of claim 5, wherein a porosity of the polishingpad is between about 10% and about 80%.
 7. The CMP tool of claim 5,further comprising a slurry dispenser, wherein the slurry dispenser isconfigured to dispense a slurry on the polishing pad during the CMPprocess, wherein a diameter of abrasives in the slurry is smaller thanabout 30 nm.
 8. A method of performing a chemical mechanicalplanarization (CMP) process, the method comprising: rotating a polishingpad at a first rotational speed; dispensing a slurry on a first surfaceof the polishing pad; pressing a wafer against the first surface of thepolishing pad, the wafer being held by a retaining ring of a carrier;and generating vibrations at the polishing pad during the CMP processusing a megasonic generator.
 9. The method of claim 8, wherein the firstrotational speed is between about 30 round per minute (rpm) and about120 rpm, wherein the method further comprises rotating the wafer at asecond rotational speed between about 30 rpm and about 120 rpm.
 10. Themethod of claim 8, wherein generating vibrations comprises generatingvibrations having a frequency between about 10 KHz and about 50 KHzusing the megasonic generator.
 11. The method of claim 10, wherein aporosity of the polishing pad is between about 10% and about 80%. 12.The method of claim 11, wherein a diameter of abrasives in the slurry issmaller than about 30 nm.
 13. The method of claim 12, wherein dispensingthe slurry comprises dispensing the slurry on the first surface of thepolishing pad during the CMP process at a flow rate between about 0.1liter per minute and about 0.5 liter per minute.
 14. The method of claim12, wherein the carrier comprises a membrane that is in contact with thewafer during the CMP process, wherein pressing the wafer comprisesinflating the membrane at a pre-determined pressure level to press thewafer against the first surface of the polishing pad, the pre-determinedpressure level being between about 0.5 pound per square inch (psi) andabout 3 psi.
 15. The method of claim 8, wherein generating vibrationscomprises: generating the vibrations at a first vibration frequencyduring a main polish step of the CMP process; generating the vibrationsat a second vibration frequency smaller than the first vibrationfrequency during a buffing polish step of the CMP process; andgenerating the vibrations at a third vibration frequency smaller thanthe second vibration frequency during a de-chuck step of the CMPprocess.
 16. The method of claim 8, wherein generating the vibrationscomprises generating a first burst of vibrations and a second burst ofvibrations separated from the first burst of vibrations by a period oftime with no vibration, wherein a first amplitude of the first burst ofvibrations is different from a second amplitude of the second burst ofvibrations.
 17. A method of performing a chemical mechanicalplanarization (CMP) process, the method comprising: holding a wafer by aretainer ring attached to a carrier; pressing the wafer against a firstsurface of a polishing pad, the polishing pad rotating at a first speed;dispensing a slurry on the first surface of the polishing pad; andgenerating vibrations at the polishing pad.
 18. The method of claim 17,wherein generating the vibrations comprises generating the vibrationswith a vibration frequency between about 10 KHz and about 50 KHz using amegasonic generator attached to the carrier, the megasonic generatorcomprising a piezoelectric transducer.
 19. The method of claim 17,wherein the first speed is between about 30 round per minute (rpm) andabout 120 rpm, and the method further comprises rotating the carrier ata second speed between about 30 rmp and about 120 rpm.
 20. The method ofclaim 17, wherein generating the vibrations comprises generating thevibrations with a first vibration frequency during a first stage of theCMP process, and generating the vibrations with a second vibrationfrequency different from the first vibration frequency during a secondstage of the CMP process.